Tank manufacturers publish standing losses in terms of energy lost over 24 hours, but I’d like to see how that translates into temperature changes in tanks, and in the real world at that.
Does anyone have charts of DHW tank temperature where the tank has been heated to design temperature and left standing for 24 hours or more? Would be good if you could share the size of the tank too.
This is a Sunex Fish 300l tank during my holidays last year. It wasn’t quite fully heated, but this should give you an idea. Temperature in the heating room is also included.
This is the best I have for my 180L tank.
Note that I do not trust this temperature in the slightest, its just showing the temp at one particular point in the tank. Some times a single shower can cause it to drop 20C in a matter of minutes other times 5C.
I’ve had perfectly hot showers when the tank has been showing 15C
Hi John, how big is that tank? Thanks
sorry should have said 180L
John
Here’s how my DHW tanked cooled down. The tank is a Rheem Proterra Performance Plus heat pump hybrid heater with a volume of 50 gallons (189 liters).
This is from December, when the ambient air temperature was 55°F (12.8°C)
And here in June when ambient air was 66°F (18.9°C)
I did another experiment during a short holiday. Charged the tank to ~65°C and let it drop over three days. Here’s the result for the center sensor
and here for the top sensor
An exponential fit yields 3.40 W/K (T_{1/2} = 2.96 days) for the center sensor and 3.11 W/K (T_{1/2} = 3.24 days) for the top sensor at an environmental temperature of 18.5°C. The tank is a Sunex Fish S15 300X (300l).
The decay is not fully exponential, especially for the center sensor there is some “wobble”. This is probably due to the fact that the tank is not at a single temperature so there’s heat conduction through the layers of hotter/colder water on top of the external cooling.
Nice feature additions @Andre_K, both Calculate dhw standby loss by andrekuehne · Pull Request #205 · emoncms/app · GitHub and Add DHW data to myheatpump app by andrekuehne · Pull Request #204 · emoncms/app · GitHub. Both merged and added to emoncms.org.
Im getting 1.3-1.5 W/K on my 150L tank 
Wow that’s really good, factor 2 below mine. It’s going to be interesting seeing more of a crossection on different systems.
Edit:
Thinking about it, this might indicate that my DHW circulation pump might be faultly and allow gravity driven circulation when off. I’ll have to investigate this.
